Coating metals and other materials with oxide and articles made thereby



May 3, 1955 w. M. WHEILDON, JR 2,707,591

COATING METALS AND OTHER MATERIALS WITH OXIDE AND ARTICLES MADE THEREBY Filed Sept. 2, 1953 2 Shegts-Sheet l V 36 I INVENTOR.

WILL/AM MAXWELL WHE/LDONJE H 23 TTo/ENE y 3, 1955 w. M. WHE!LDON, JR 2,707,691

comma METALS AND OTHER MATERIALS WITH OXIDE AND ARTICLES MADE THEREBY Filed Sept. 2, 1953 2 Sheets-She et 2 JNVENTOI? WILL/AM MAXWELL WHElLDO/V J4.

7 :r /MyC+M-F-7 ATTO Y United States Patent COATING METALS AND OTHER MATERIALS WITH OXIDE AND ARTICLES MADE THEREBY Application September 2, 1953, Serial No. 378,091 17 Claims. (Cl. 117-104 The invention relates to the coating of metals and other materials, such as graphite, with oxide. Many diiferent oxides and combinations of oxides, including silicates, can be used. This application is a continuation-in-part of my copending application Serial No. 303,364, filed August 8, 1952, and now abandoned.

One object of the invention is to provide a wear resistant surface on metal or other material. Another object of the invention is to provide a refractory surface on metal or other material. Another object is to provide metal or other material with a coating protecting it from oxidation.

Another object of the invention is to produce components for gas turbines, jet engines, rocket nozzles and chambers and other components which are subjected to hot flame in use, by casting, forging, spinning, turning or machining the components to the desired shape (which may be complex) out of materials such as metal and graphite which can easily be wrought and thereafter coating the components with an oxide or oxide combination which is resistant to the hot flame, resistant to attrition by friction, oxidation and other chemical reaction and is also highly refractory, whereas the underlying metal or graphite does not have these properties.

Another object is to provide a dense low-porosity wearresistant protective oxide coating in which the grains of molten oxide bond together to form a continuous homogeneous coating which is bonded tenaciously to a base member without the use of a separate bonding agent. Another object is to provide an oxide coating having a wear resistant surface with or less of pores. Another object is to provide a spray coated oxide coating having a surface roughness of around 350 micro-inches which may be ground to a highly finished surface of 20 to micro-inches.

Another object is to provide a thermal insulation coating on a piece of metal. Another object is to provide an electrical insulation coating on a piece of metal. Another object is to improve the surface resistance to hot gases of ceramic parts designed for high temperature use. 4 I Another object is to produce an aluminum oxide coated article of superior heat resistance. Another object is to produce a zirconium oxide coated article of superior heat resistance.

Another object is to provide a method of the type indicated which can be readily carried out with simple equipment and requiring no especial skill. Another object is to provide a method for coating material with oxide or oxides which does not require the preheating of the material.

Other objects will be in part obvious or in part pointed out hereinafter.

In the accompanying drawings illustrating one type of fusing and spraying gun which can be used to coat materials with oxide according to the invention,

Figure 1 is a side elevation of the gun, a portion of the casing being broken away to show the feeding mech anism,

Figure 2 is a sectional view taken along the line 2--2 of Figure 1,

Figure 3 is a simple electrical diagram illustrating how the rate of feed can be varied.

Figure 4 is a photomicrograph of a section of an alumina coating on a piece of steel, the coating of alumina being from about .016 inch to .019 inch thick and having been made in accordance with this invention.

As conducive to a clearer understanding of the present invention it is noted that the mechanism for carrying out the coating process can have many different forms and is not the subject of my invention. For many years ma terials of various kinds have been coated with metal using metal spraying guns which apparently were originally developed in Switzerland. See for example, U. S. Patent No. 1,100,602 to E. Morf of Zurich, Switzerland, dated June 16, 1914. These guns were known as Schoop metal spraying guns and they were provided with means for feeding metal wire at a steady but variable rate, means for fusing the wire and means for atomizing the fused metal and projecting it as a spray. The name was derived from M. U. Schoop of Hongg, Switzerland, who developed the process; see his U. S. Patent No. 1,128,058 in which, however, the metal is supplied to a blast of air in molten condition.

Years ago attempts were made to spray molten oxides in similar manner. See for example U. S. Patent No. 1,268,030 to J. P. A. McCoy dated May 28, 1918. McCoy provided oxide in finely divided condition, that is to say as a powder, which he dropped into an oxyacetylene or oxyhydrogen flame thus to produce a spray of molten oxide. He said that in order to coat a metallic body with oxide the metal had to be heated before being coated. I have found that, if the metal is heated to a good red heat, no coating at all is formed. If the piece of metal is at room temperature the coating formed by the McCoy process is not an integral coating, is not a coating integrally bonded to the underlying metal or other base member and on the contrary is only a non-rigid coating which is weak and quite non-resistant to attrition, in fact it can be picked off with the fingernail.

I have discovered that a tenacious coating of oxide, for example alumina, zirconia, or spinel can be formed on metal or graphite and on other substances by feeding a solid rod of such oxide into a high temperature blast of gas to fuse the oxide, to atomize it and to spray it, the gas having suflicient velocity to atomize the molten oxide and to spray it on the metal or graphite or other substance, and that proceeding in this manner the metal or graphite or other substance does not have to be heated before the coating is applied. Furthermore my coating will not craze, crack, spall or peel.

I preferably provide rods of sintered alumina, zirconia, zircon, spinel etc., that is to say the rods are of uniform composition made by sintering small particles together under high temperatures. Taking finely divided oxide material and providing a temporary binder, small diam" eter rods can be molded under pressure in a manner now well known to the art, and thereafter the rods can be heated to a temperature to sinter the particles together while at the same time burning out the temporary binder. Strong refractory rods of pure oxide material are thus produced. But this technique is not new in itself being 0 sintered rods, rods bonded together with a glassy phase can be used.

The bonding or sintering operation of forming the rods is carried out at a temperature that will depend on the rod composition being made. The objective is to produce a strong well bonded rod, substantially free from organic matter and water or other volatile constituents. In all cases the firing operation employed for maturing the bonding of the rods is carried out at temperatures of at least a dark red heat, and usually temperatures of 1000 C. or more are employed.

As an illustrative example of the process of the invention, I procured a metal spray gun constructed substantially in accordance with U. S. Patent No. 2,227,752 to H. S. Ingham of January 7, 1941. These guns are readily available on the market. I removed the turbine mechanism and substituted a variable speed motor connected to the gun by a flexible drive. I also provided new feeding Wheels more adaptable to feeding rigid rods than the original wheels intended for feeding flexible wire.

Referring now to Figure 1, the motor had a base 11 secured to a table, not shown, and the gun had a grip 12 by means of which it could be held or clamped in place if desired. The motor 10 was connected to the input shaft 13 (see Figure 2) of the spray gun bymeans of a flexible drive shaft 14 enclosed in a flexible tube 15.

The shaft 13 had a worm, not shown, driving a worm substituted rubber wheels 26 and 27 to feed the oxide rods 30 which were one-eighth inch in diameter. The gun was connected to an air hose 31 and an acetylene conveying hose 32 and an oxygen conveying hose 33. When the flame was lighted the rod 30 issuing through the nozzle 35 was fused. Referring to Figure 3, the rate at which the oxide rod was fed through the gun could be varied by a rheostat 36 in the circuit for the motor 10. I had success using alumina rods and spraying them at a feeding rate of 1.18 inches per minute and fusing and spraying zirconia rods at a feeding rate of 1.456 inches per minute. In each case the rods were one-eighth inch in diameter. The rods actually used in the large scale operations were sintered rods.

It is unnecessary for me to describe the further mechanical details of the gun which I used as any other similar apparatus can be used and the details of this particular one will be found to be fully described in the above referred to patent to :Ingham which clearly explains all the features of construction.

Any kind of metal can be coated in accordance with In other words the object being coated should be held just off the end of the flame. With this proviso if coating doesnt appear the gun should be held a little closer. Usually the correct distance of the tip of the nozzle 35 from the object to be coated is between one and two inches.

Coating of any thickness can be applied it being merely a matter of how much time is taken. As for gas pressures I find it is satisfactory to have the oxygen at 15 to 20 pounds, the acetylene at 15 to 20 pounds and the air at 60 pounds per square inch.

The above figures are for coating with alumina; in the case of zirconia the oxygen and acetylene are preferably at the same pressure but the air pressure is preferably 'at about 40 pounds per square inch.

For many purposes it is advantageous to have coatings If the metal or graphite or other H thicker than .004, although thinner coatings are cheaper to apply and; when they increase the service life of the piece sufliciently, compared to no coating, they too are useful.

For coating objects with spinel or other oxides, the air and acetylene pressures can be the same, the feed can be varied until a good coating is obtained, and for materials fusing below 2000 C. an air pressure of about 60 pounds would be preferred whereas for materials fusing above 2000 C. the pressure should be a little lower. As above stated the pressure for zirconia is found to be preferably at 40 pounds per square inch and as zirconia fuses at 2700" C., a graph of the melting point of oxides on this slope will quickly give the answer as to what pressure should be used.

I have mentioned coating material with alumina, zirconia and spinel. Alumina coatings and zirconia coatings are especially desirable for many practical purposes.

Spinel is R'O-R2"O3 in which R can be any of magnesium, iron, zinc, manganese and nickel and R? can be any of aluminum, iron and chromium. The commonest and best known type is mangesiurmaluminum spinel. Any of these or complexes thereof can be used. For example many pairs of spinels form solid solutions with each other in all proportions. The aluminum spinels form a series all of the members whereof are miscible with each other. The iron spinels in which the iron oxide is F6203 form another series all of the members of which are miscible with each other, and so do the chromium spinels. Furthermore rods made of sintered mixtures of the spinel oxides in the right proportions can be used in the invention and the coating will be a true spinel. The spinel oxides are soluble in the spinels so that the spinel can contain excess of a spinel oxide. This is especially true of alumina which can be readily dissolved in the aluminum spinels.

I can coat with any refractory stable metal oxide and in this connection I use the widely accepted definition of a metal which is that it is a good conductor of electricity and forms a basic or amphoteric oxide. This definition excludes silicon, germanium, boron, carbon, phosphorous, arsenic and sulphur. Suitable oxides (leaving out very expensive and very rare metal oxides although they too can be used) are aluminum oxide, barium oxide, beryllium oxide, calcium oxide, iron oxide, manganese oxide, nickel oxide, strontium oxide, thorium oxide, titanium oxide, uranium oxide, and zirconium oxide. Many of these metals have more than one oxide and all can be used which are refractory and stable. Those oxides are stable which will not be reduced to metal by fusion in the flame or volatilize to any objectionable extent. By refractory I mean having a melting point over l000 C.

Silica is not in the list as silica is glass forming and when fused and blasted forms fibers. But a considerable ercentage of silica can be tolerated, for example emery or zircon (ZrOz'siOz) can be used. The process can be carried out satisfactorily to produce excellent articles if the major portion (by weight) of the material used is refractory stable metal oxide and the balance of the material is compatible therewith. Thus any amount of silica constituent can be used up to by weight. Mixtures and solid solutions as well as double, ternary and quarternary oxides can be sprayed in accordance with the invention. For example discrete particles of alumina and zirconia can be si-ntered together to make a rod which can be used.

It is preferred, when mixtures consisting essentially of oxides are employed, that the melting point of compounds, eutectics or glasses corresponding to each com- Any of the following minerals in rod form can be used to make the coating:

Chromite FeO-CrzOa.

Chrysoberyl BeO-AlzOa.

Emery Mix. of corundum, magnetite,

hematite, quartz and spinel.

Gahnite ZnAl204.

Geikielite (Mg,Fe)O-TiO2.

Hercynite FeAl2O4.

Ilmenite FeO-TiOz.

Picotite (Mg,Fe)O-(Al,Cr)2Os.

Pleonaste (Mg,Fe)O'Al2Os.

Pseudobrookite 2Fe2O3-3Ti02.

Pyrochlorite RNb2Os-R(Ti,Th)O3.

Spinel MgO-AlzOs.

Tantalite (Fe,Mn)[(Cb,Ta)O3]z.

Tapiolite Fe(Ta,Nb)2Oe.

Thorianite (ThU)Oz(+He,Ce,La,Pb,Fe).

Uraninite UO3'UO2,PbO.

Zircon ZrOz-SiOz.

Those minerals containing water of crystallization will lose this Water on being sintered into rod form but they are nevertheless usable in the invention. Many other combinations of oxides, mixtures, solid solutions and compounds can be made and all Within the definition are usable in the invention. As will be seen from the ex amples, minor proportions of other elements such as halogen, helium etc. are not excluded from the starting material but some thereof may be eliminated during the sintering into rod form.

I made some rods out of the following composition:

Composition for vitrified alumina rods Percent by weight Powdered aluminum oxide 36 Feldspar 20 Kaolin clay 30 Ball clay l Powdered flint 4 This material was molded, using an addition of 19% by weight Water and 1% by weight sulphonated castor oil to provide the necessary workability, into rods one eighth inch in diameter and then the rods were fired under cone 16 (about 1450" C.) conditions in a kiln.

These rods were used in the spray gun with the air at 40 pounds per square inch of pressure, the oxygen and acetylene pressure being between 15 and 20 pounds each. Good coatings on sheet steel were produced. The coatings consisted of aluminum oxide with a glassy phase. This is an example of the use of rods bonded together by a glassy phase. These rods were vitrified bonded alumina rods.

I also made coatings on steel using zircon rods. Zircon of particle size 325 mesh and finer was molded with the use of 20% of a heavy starch solution as temporary binder. Rods one eighth inch in diameter were then molded and sintered at a temperature of 1450 C. Refractory coatings on steel plates were produced by the use of these rods in the spray gun. Zircon which is ZrOrSiOz, is decomposed by the spraying operation into zirconia, ZrOz, and glassy silica. This reference is another example of a coating consisting of crystalline metal oxide with a minor proportion of a glassy phase.

If the expense is justified, steel can be protected from the elements by a permanent coating of oxide applied in accordance with this invention. Whereas a coating of paint lasts only for a limited time, a coating of oxide applied according to the invention will resist attack by wind, rain and sun almost indefinitely.

By the word rod I intend to include rods square or triangular in cross section and hexagonal rods as well as round ones. Also other shapes can be used.

In the specific disclosure the molten oxide is atomized by a blast of air. This can be referred to generically as a gas since other gases would serve equally well. In some cases the combustible gas and the combustion supporting gas can have enough pressure to atomize the molten oxide. This in a sense is true in the illustrative example because the air blast assists in supporting combustion, that is to say it provides oxygen additional to that entering through the hose 33.

My method or process can also be used to coat ceramic materials. Thus crucibles can be made more refractory and less pervious by coating them with a pure refractory oxide such as alumina or zirconia. Bricks can be made more resistant to corrosion by hot gases by coating them with alumina or zirconia. Batts and kiln furniture can be given longer life by coating them in accordance with the invention.

For many applications involving high temperature use, it is desirable that the base member on which the coating is applied have a melting point about 1Q0O C. However, it is surprising that less refractory base members, such as soft solder, asbestos-board, and wood can also be coated successfully.

Ceramic material can itself be coated with oxide according to the invention. This is not a useless thing to do. For example a nitride bonded silicon carbide rocket nozzle can be coated with oxide according to the invention, for example with aluminum oxide, and thereby be rendered more oxidation resistant with resultant longer life in use. All ceramic materials can be coated in accordance with the invention. Bricks, batts and kiln furniture come under the general description ceramic materia For guided missiles, rockets and the like, parts can be made out of graphite which is refractory and then coated with oxide according to the invention thus to protect the graphite from oxidation. One advantage of using graphite is that it is easily machined.

It is well known that the turbine blades of jet engines are very expensive to make and have to be replaced at too frequent intervals. By forging refractory metal such as molybdenum a heat resistant blade which is not brittle can readily be produced while forging and polishing will produce an accurate enough blade for practical purposes. However, these blades will gradually deteriorate in the hot flame by oxidation. I can coat such blades with oxide according to this invention thus producing superior blades. Because of its refractoriness, I prefer to coat such blades with zirconia and I prefer the stabilized zirconia containing from 3% to 6% of lime made in accordance with the patent of my colleagues Ballard and Marshall, No. 2,535,526.

In one aspect of my invention, articles produced according to the invention constitute a base member having a rigid integral coating integral with said base member, said coating essentially consisting of refractory oxide the major portion of which is crystalline metal oxide.

While usually the porosity of the coating is of the order of 10%, occasionally a more porous coating is produced and a porosity up to can be tolerated for some uses. It would be desirable to produce coatings of zero porosity in order completely to stop oxidation of the underlying metal and I have produced coatings having less than 10% porosity but do not known the lowest value of porosity obtainable.

A surprising fact is that my alumina coating has been examined by X-ray spectrography and found to be alumina of the gamma-type. Gamma-type aluminas, of which gamma alumina itself is one example, have been produced by thermal dehydration of hydrates of aluminum oxide at low temperatures, so it is surprising that gamma-type alumina is produced by melting and spraying of rods composed of alpha alumina. The difference between alpha alumina and gamma alumina consists in part that alpha alumina is hexagonal in crystalline structure whereas gamma alumina is cubic in crystalline structure. Another difference is that alpha alumina has a true specific gravity of about 3.97 whereas gamma alumina has a true specific gravity of about 3.62. The index of refraction for alpha alumina is about 1.76 while for gamma alumina it is close to 1.70. Alpha alumina polarizes light to some extent whereas gamma alumina does not. Alpha alumina is anisotropic whereas gamma alumina is isotropic.

In the actual practice of the invention I have obtained the best results so far with the use of alumina rods. These in most cases have been alpha alumina sintered rods.

The next most useful material to form the coating is probably zirconia. Zireonia is highly refractory. Zirconia as commercially available always contains a minor proportion of hafnia and obviously I use the commercial- 4 ly available material. I have used zirconia stabilized with about of lime. This material, was crystalographically, partly monoclinic and partly cubic, but surprisingly the coatings were found to be 100% cubic. The advantage of stabilized zirconia is that it can be subjected to repeated cycles of heating and cooling without cracking.

In stating that the coating is integral with the base member, I mean that the adhesive strength between thecoating and the base member is at least substantially equal to the cohesive strength of the coating. In stating that the coating is integral, I mean that the individual particles are self-bonded together to produce a coating such that the coating itself constitutes a rigid structure independent of the backing. All of the refractory metal oxides per se, where metal is defined as above, are crystalline whereas fused silica is amorphous.

Figure 4 shows a photomicrograph of a coating of alumina on steel, shown in cross section. This section was prepared by imbedding the coated steel in a phenolic resin mounting and then sectioning and polishing. In the picture the steel 1 is white, the alumina coating 2 is light gray and the phenolic resin 3 is dark gray with a black line of separation 4 because of the polishing operation. The integral bond with the roughened steel is readily apparent. The structure of the alumina coating shows interlocking laminations 5 as a result of the method of application. The black spots 6 in the alumina are for the most part holes that were produced by the polishing operation, and were not originally in the alumina.

I coated a piece of black iron sheet with alumina by the process of subject application. The coating was about .025 inch thick. This coating was subjected to an oxyacetylene flame which heated the coating to so high a temperature that the coating freely flowed in one spot. The temperature certainly wasat or above the melting point of alumina which is now believed to be close to 2015 C. The black iron sheet did not melt nor appreciably oxidize nor did the coating crack significantly. Slight cracks occurred on return to room temperature but the coating is still strongly adherent and would protect the iron for many repetitions of the same test.

Repeatedly I have found that pieces of iron and steel coated according to my process of subject application can be bent without destroying the coating or injuring it to any practical extent. Minor cracks do not matter because the coating will still withstand rough mechanical abuse and temperatures up to the melting point of the oxide involved.

In order to test the mechanical strength of a .025 inch thick coating of alumina, sample plates 2" X 2" x thick steel were coated. These plates, supported on a flat rigid surface, were subjected to impact from a one and a quarter inch diameter steel ball dropped from various heights. Drops from three feet showed no appreciable indentation to the coated surface. and fourteen feet showed minor indentations with a slight powdering of the alumina at the surface but no cracking or other destruction of the coating. One drop of the same ball from 57 feet showed an appreciable indentation and powdering of the surface but still left an undisturbed Drops from six feet structure below the powdered surface. Stress-was transmitted sufficiently to show as a raised portion on the back of the plate.

Typical physical properties of coatings according to my process of subject application, using alumina, are as follows:

Composition: about 99% aluminum oxide.

Bulk Density: 3.2 grams per cubic centimeter.

Porosity: from 8% to 11% open pores and from 0% to 1% closed pores.

Corrosion Resistance: excellent.

Thermal Shock Resistance: excellent especially on the inside of a cylinder or the like. Where the surface coated is concave, the coating has an interference fit with almost any metal and is mechanically practically integral with the metal.

Thickness as now available: from 0.010" to 0.050". However, I can make coatings much thicker than 0.050" and have actually made coatings as thick as half an inch. Obviously there is hardly any minimum thickness since the thickness is a function of the time during which the gun is applied to the piece to be coated.

Thermal conductivity: estimated to be twenty British thermal units per hour per square foot, inches per degree Farenheit at about 1600 F.

Electrical conductivity: non-conducting.

Total emissivity: estimated to be 0.3.

Temperature resistance: the melting point of alumina as above given.

It will thus be seen that there has been provided by this invention a method of coating metals and other materials with oxide and articles produced thereby in which the various objects hereinabove set forth together with many thoroughly practical advantages are successfully achieved. As many possible embodiments may be made of the above invention and as many changes might be made in the embodiments above set forth, it is to be understood that all matter hereinbefore set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

I claim:

1. Process for coating solid material with stable metal oxide comprising feeding a rod the major portion of which consists of stable metal oxide said rod having been matured by firing at at least a dark red heat into a flame hot enough to melt the metal oxide to form molten metal oxide and instantaneously atomizing the molten metal oxide to form discrete molten particles of said oxide and coincidentally projecting said molten particles with a blast of gas onto a surface which is so close to the locus of melting and atomizing said metal oxide that said molten particles are still molten metal oxide particles upon impacting said surface.

2. Process according to claim 1 in which the major portion of the rod is crystalline metal oxide.

3. Process according to claim 1 in which the rod is a sintered rod.

4. Process according to claim 3 in which the stable metal oxide in the rod is alumina.

5. Process according to claim 1 in which the stable metal oxide in the rod is alumina.

6. Process according to claim 1 in which the rod is zircon. I

7. Process according to claim 1 in which the stable metal oxide in the rod is zirconia.

8. As a new article of manufacture, a solid rigid base member having a rigid integral coating of interlocking laminate structure integral with said base member, said coating essentially consisting of refractory oxide the major portion of which is crystalline metal oxide, the adhesive strength between the coating and the base member being at least substantially equal to the cohesive strength of the coating, and the individual particles of -the coating being self bonded together so that the coatpoint over 1000 C. and said article having been made by the process of claim 1.

9. An article according to claim 8 in which the base member is a piece of metal.

10. An article according to claim 9 in which the oxide essentially consists of alumina.

11. An article according to claim 10 in which the alumina essentially consists of gamma type alumina.

12. An article according to claim 8 in which the oxide essentially consists of alumina.

13. An article according to claim 12 in which the alumina essentially consists of gamma type alumina.

14. An article according to claim 8 in which the oxide essentially consists of crystalline zirconia.

15. An article according to claim 14 in which the base member is a piece of metal.

16. An article according to claim 8 in which the coating essentially consists of zircon.

17. An article according to claim 16 in which the base member is a piece of metal.

References Cited in the file of this patent UNITED STATES PATENTS 1,299,988 Metzger Apr. 8, 1919 1,978,415 Collins Oct. 30, 1934 2,231,247 Bleakley Feb. 11, 1941 2,423,490 Erhardt July 8, 1947 FOREIGN PATENTS 315,343 Great Britain Jan. 31, 1930 234,969 Great Britain June 11. 1925 

1. A PROCESS COATING SOLID MATERIAL WITH STABLE METAL OXIDE COMPRISING FEEDING A ROD THE MAJOR PORTION OF WHICH CONSISTS OF STABLE METAL OXIDE SAID ROD HAVING BEEN MATURED BY FIRING AT AT LEAST A DARK RED HEAT INTO A FLAME HOT ENOUGH TO MELT THE METAL OXIDE TO FORM MOLTEN METAL OXIDE AND INSTANANEOUSLY ATOMIZING THE MOLTEN METAL OXIDE TO FORM DISCRETE MOLTEN PARTICLES OF SAID OXIDE AND COINCIDENTALLY PROJECTING SAID MOLTEN PARTICLES WITH A BLAST OF GAS ONTO A SURFACE WHICH IS SO CLOSE TO THE LOCUS OF MELTING AND ATOMIZING SAID METAL OXIDE THAT SAID MOLTEN PARTICLES ARE STILL MOLTEN METAL OXIDE PARTICLES UPON IMPACTING SAID SURFACE. 